1. Field of the Invention
This invention relates to an open-center three-way hydraulic valve for selectively connecting a utilization circuit either to a high-pressure source (admission position) or to a low-pressure tank (drain position), said valve being actuated hydraulically by a hydraulic pilot jack which is put into communication via a control circuit either with a high-pressure source or with a low-pressure tank so as to bring the valve into one position or the other.
A valve of this type finds a particularly important application in the case of hydraulic control of high-voltage electric circuit-breakers. As a rule, the valve is provided in this case with a so-called "self-maintaining" hydraulic circuit for applying a transient pressure signal to the aforesaid control circuit of the pilot jack. The function of this self-maintaining circuit is to maintain the valve in the position (admission or drain position) to which it has been brought by the transient pressure signal aforesaid after disappearance of this pressure signal.
2. Description of the Prior Art
The valve considered in the foregoing has been described in French Pat. No. 1,098,565 filed on Jan. 15th, 1954 (or in U.S. Pat. No. 2,900,960) and has come into standard use for the hydraulic control of high-voltage electric circuit-breakers.
Since a valve of this type has to be absolutely leak-tight in both the admission position and the drain position, the valve does not have any slide-valves but is provided with closure members or so-called valve-heads, namely an admission valve-head and a drain valve-head.
In the conventional design, it is necessary in the interest of good preservation and reliability to ensure that the admission and drain valve-heads are perfectly guided so that application of the closure members against their respective valve-seats should always take place on the same imprints. As a result, the two valve-heads are physically independent and interact with each other solely by exerting a thrust force. The admission valve-head which is movable on the high-pressure supply side with respect to the valve-seat is displaced by a spring and the drain valve-head on the low-pressure supply side with respect to its valve-seat is displaced by said hydraulic pilot jack.
Thus in the event of lack of pressure in the high-pressure source, the admission valve-head is closed under the action of its spring and the drain valve-head is opened. In the event of pressurization of the high-pressure source, the valve is leak-tight and does not prevent pressurization since the admission valve-head is closed and there is no communication between the high pressure and the drain tank.
In the case of application to the control of high-voltage circuit-breakers, a particular feature of these valves lies in the fact that, at the time of initiation of a circuit-breaker trip which corresponds to the drain position, the operating time-delay must be as short as possible, namely of the order of one millisecond. In consequence, the distance of travel between the closed drain position and the closed admission position is of the order of a few tenths of a millimeter (of the order of one-quarter of the diameter of the orifice of the valve-seats), which also has the effect of reducing leakage during transition through the intermediate positions between the admission position and the drain position (valve with open center).
In conclusion, in the event of disappearance of pressure in the high-pressure source, a conventional valve of this type naturally takes up a stable position which is the "drain" position.
In many hydraulic circuit-breaker-control systems, a differential jack is employed for actuating the moving contact of the circuit-breaker. In a jack of this type, the annular jack chamber is continuously subjected to the high pressure of the source in order to urge the circuit-breaker continuously toward its open position. One example of a hydraulic control system of this type has been described and illustrated in French Pat. No. 2,317,532 filed on July 7th, 1975 (or in U.S. Pat. No. 4,026,523).
However, as will be seen in greater detail in the description which now follows, when this known type of valve is employed for controlling the supply/discharge of a differential jack of this type, operation of the circuit-breakers is subject to certain disadvantages (low-speed operation) in the event of a decrease or a reduction to zero of the pressure in the pressure source.
It is therefore necessary as a general rule to provide a system which initiates automatic tripping of the circuit-breaker in the event of a fall in pressure below a predetermined threshold value so as to avoid any low-speed operation of the circuit-breaker.
In certain electric power distribution systems, however, automatic trips of circuit-breakers are found unacceptable and it is preferred on the contrary to maintain the circuit-breakers in the closed position.
It is for this reason that systems have been provided for locking the circuit-breakers themselves in the closed position but this is complicated from the point of view of construction and in fact has the effect of reducing safety. Devices have also been proposed for ensuring in certain cases that the valve is prevented from returning to its rest position (drain position corresponding to the open position of the circuit-breaker), particularly in the event of a drop in supply pressure. Devices of this type, however, are difficult to construct and fail to offer reliable operation, particularly on account of the short distance of travel of the valve-heads as mentioned earlier.
The aim of the present invention is to overcome these drawbacks by means of a three-way valve which has two stable positions, namely a drain position and an admission position.